Web-based viruses decreased by half in September, according to new figures from security firm ScanSafe. 

But, while instances of web viruses fell 47 per cent, spyware rose 21 per cent during the month.

ScanSafe's Global Threat Report is based on the real-time analysis of more than five billion web requests and more than 10 million web threats processed by the company in September.

"We were surprised to see such a drop-off in web viruses during September," said Eldar Tuvey, chief executive and co-founder of ScanSafe.

"With all the recent attention on Microsoft vulnerabilities and zero-day exploits, we thought we would see an increase. But, despite the hype, a mass outbreak did not occur in September.

"In fact, none of the top 10 web viruses blocked by ScanSafe during the month was an exploit of a Microsoft vulnerability uncovered in September, including the much publicised Vector Markup Language flaw."

Reports surfaced on 20 September of a new vulnerability in Internet Explorer specifically targeting a Windows component called vgx.dll.

This component is meant to support VML documents in Windows. VML is used for high-quality vector graphics on the web and for viewing pages in Internet Explorer.

ScanSafe blocked 158 unique viruses during the month, 31 per cent of which were new unique viruses blocked for the first time by ScanSafe.

Zero-hour threats, attacks that appear before an antivirus signature is available, accounted for 14 per cent of all web viruses blocked by ScanSafe in during the month.

Spyware and adware increased 21 per cent in September and eight per cent of the spyware blocks were to prevent already infected PCs from "calling home", or transmitting outbound data to a spyware domain.

"An effective anti-spyware solution should filter both inbound and outbound web traffic to identify infected PCs for remediation and block them from being further compromised," said Tuvey.

September's results do not mirror August's when web viruses rose 23 per cent while spyware decreased 12 per cent.

"There is a seasonality to web viruses and spyware. We fully anticipate a jump in malware as consumers go online to start their holiday shopping," said Tuvey.

"As such, we caution users and those responsible for corporate networks not to be lulled into a sense of complacency, but to take the necessary precautions to protect themselves from web threats."